Reagent for forming an insulating coating on the surface of electrical steel sheets

ABSTRACT

REAGENT FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICAL STEEL SHEETS IS DISCLOSED WHEREIN ALUMINUM NITRATE AND/OR AMORPHOUS ALUMINUM HYDROXIDE ARE/ IS INCORPORATED INTO AN AQUEOUS SOLUTION OF PHOSPHATE COMPOUNDS IN AN AMOUNT OF 0.4-2.1 PARTS BY WEIGHT BASED   ON ALUMINUM. THE AQUEOUS SOLUTION OF PHOSPHATE COMPOUNDS INCLUDES 100 PARTS BY WEIGHT OF MONOBASIC MAGNESIUM PHOSPHATE AND 5 TO 30 PARTS BY WEIGHT OF CHROMIC ANHYDRIDE.

March 14, 1972 rrsuo M EIAL 3,649,372

[(EAGIuNT FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICALSTEEL SHEETS Filed Aprll 16, 1969 4 Sheets-Sheet 1 I C -x :AJZ(NO3)39H2O P (On gloss film) Imermminafion resis1oncen cm lominoflon .p

I 20 I I |o l I l I l A2(NO3)39HzO 0 IO 20 so 40 l l l I l March 14,1972 MITSUO IMAI EFAL 3,649,372

REAGLLINT FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICALSTEEL SHEETS filLecl April. lb, 1969 4 Sheets-Sheet 2 (Good) Z 6 B- Q. 8a. e CrOa 6gr g f I CrOa 3gr g g CrOs L5gr 2 h 1 CrOa Ogr Amount ofA(NO3)3 9H2O 2 4 6 9 added To reagent gr A 2 g/Mg (H2PO4)2 lOOgr March1972 MITSUO IMAI E L 3,649,372

REAGENT FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICALSTEEL SHEETS Fned April 16, 1969 4 Sheets-Sheet 5 A! (0H)3 AJI(NO3)39H2O A 0 No flowing m M M m u u m M m u u M U m M M M M M w w ww m.

llll ANNOa); 9H2O 0 March 14, 1972 MITSUO IMAI ETAL 3,649,372

RLAUQNF FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICALSTEEL SHEETS u'lleu Aprn lb, 1969 4 Sheets-Sheet 4FIIIIIIIIIIIIIIIIIIIIIIIA United States Patent U.S. Cl. 1486.16 9 ClaimsABSTRACT OF THE DISCLOSURE Reagent for forming an insulating coating onthe surface of electrical steel sheets is disclosed wherein aluminumnitrate and/ or amorphous aluminum hydroxide are/ is incorporated intoan aqueous solution of phosphate compounds in an amount of 0.4-2.1 partsby weight based on aluminum. The aqueous solution of phosphate compoundsincludes 100 parts by weight of monobasic magnesium phosphate and to 30parts by weight of chromic anhydride.

This invention relates to reagent for forming an insulating coating onthe surface of electrical steel sheets and particularly to improvementson a reagent for use in a method of coating and baking phosphatesolution on the surface of electrical steel sheets and providing aninsulating coating having various excellent properties.

The insulating coating formed on the surface of electrical steel sheetsis required to have a number of different properties in dependence withthe magnetic property of the electrical steel sheets and the usesthereof.

For example, a grain oriented silicon steel sheet for use in wound typetransformers is required to have a large space factor, highinterlamination resistance and high heat resistance. Particularly, inthe wound core type transformer in which the iron core is impregnatedwith resin, the steel sheet is required to have an insulating coatinghaving an excellent adhesive strength for an adhesive agent, such asepoxy resin. Moreover, the grain oriented silicon steel sheet for use intransformers is required to have corrosion resistance against thetransformer oil and non-inflammable synthetic insulating oil.

A non-oriented silicon steel sheet for use in domestic electricalmachines and apparatuses is required to have excellent punchability,weldability and corrosion resistance etc., more than the above mentionedproperties. The non-oriented silicon steel sheet for use in a motor corefor refrigerators must resist against a refrigerant such as Freon gas.

Heretofore, a number of methods of forming an insulating coating on thesurface of electrical steel sheets with the aid of phosphate or chromatehave been proposed.

In the method of forming an insulating coating on the surface ofelectrical steel sheets with the aid of chromate it is necessary to usean organic compound as a reducing agent, with the result that theorganic compound becomes occluded in the insulating coating, that whenthe insulating coating is formed on the surface of a low carbonelectrical steel sheet, carburization occurs when effecting a stressrelief annealing step thus deteriorating its magnetic property, thatthere is risk of the insulating coating being deteriorated by therefrigerant such as Feron gas, and that blow holes are liable to occurin weld beads formed by a TIG-arc welding.

3,649,372 Patented Mar. 14, 1972 The method of forming an insulatingcoating on the surface of electrical steel sheets with the aid ofphosphate is not capable of easily providing an insulating coatinghaving all of desired properties and beautiful surface appearance whichis worth marketing.

This method makes use of a baking step under extremely strict conditionsin order to produce a thin and smooth insulating coating having highcorrosion resistance and insulating property, so that it is not suitablefor producing industrially stable insulating coating on a massproduction basis.

Moreover, the conventional phosphate coating has the disadvantage thatdeterioration to the adhesion of the coating occurs when it is subjectedto a stress relief annealing step and hence the coating becomes flakedoff from the steel sheet, and the interlamination resistance becomesremarkably decreased, and cannot be applied to grain oriented siliconsteel whose insulating property must be high.

An object of the invention is to obviate the above mentioneddisadvantages and provide a novel reagent for forming a coating havingexcellent properties such as insulating property, adhesive property,corrosion resistance, space factor, heat resistance, machinability,weldability, etc., in an industrially easy manner, which can be appliedeffectively to the formation of an insulating film having a high breakdown voltage on a glass film formed on a grain oriented silicon steelsheet.

A feature of the invention is the provision of such an improved reagentfor forming an insulating coating on the surface of electrical steelsheets which comprises an aqueous solution consisting of parts by weightof monobasic magnesium phosphate and 5 to 30 parts by weight of chromicanhydride, and 0.4 to 2.1 parts by Weight based on aluminum of aluminumnitrate and/or amorphous aluminum hydroxide.

The surface condition of the electrical steel sheets on which thereagent according to the invention is coated must be cleaned preliminaryto the coating treatment, but it is not necessary to apply special stepssuch as to remove a thin oxide film formed by an annealing step on thesurface of the electrical steel sheets.

Several reagents consisting of magnesium phosphate and chromic anhydrideor consisting of phosphoric acid, magnesium oxide and chromic anhydridehave heretofore been proposed. But, such reagents could not provide aninsulating coating having sufficient interlamination resistance andadhesive property.

The addition of specially defined aluminum compounds to a mixture ofmagnesium phosphate and chromic anhydride with a given mixing ratio inaccordance with the invention results in a reagent that can obtain aninsulating coating having remarkably improved interlamination resistanceand adhesive property.

In accordance with the invention the aluminum compounds are limited toaluminum nitrate and/or amorphous aluminum hydroxide. Experimentationhas shown that aluminum compounds other than aluminum nitrate and/oramorphous aluminum hydroxide or crystalline aluminum hydroxide can notattain the object of the invention.

Three kinds of aluminum hydroxide that is, monoclinic aluminum hydroxideccAl(0H) hexagonal aluminum hydroxide fiAl(OH) and amorphous aluminumhydroxide gel are well known. The amorphous aluminum hydroxide isaluminum hydroxide gel available in the market as medical antiacid.

Aluminum nitrate can be aluminum nitrate nonahydrate Al(NO -9H O. It isa matter of course that an aqueous solution of Al(NO may also be used asaluminum nitrate.

It is preferable that to 100 parts by weight of monobasic magnesiumphosphate is added to 30' parts by weight of aluminum nitratenonahydrate Al(NO -9H O or 1 to 6 parts by weight of aluminum hydroxide.

Other objects, features and advantages of the inven tion will becomeapparent from a consideration from the following specification, when thespecification is considered in conjunction with the accompanyingdrawing, in which:

FIG. 1 shows curves which explain the relationships between the amountof aluminum compounds added and interlamination resistances;

FIG. 2 shows curves which explain the relationships between the amountof aluminum compounds added and adhesive properties;

FIG. 3 shOWS a graph which explains the relationships between the amountof aluminum compounds added and baking times at 450 C.; and

FIG. 4 is a diagrammatic illustration of a circuit arrangement formeasuring break down voltage of an insulating coating formed on thesurface of an electrical steel sheet by a reagent according to theinvention.

Referring now to FIG. 1, chain line curve a, full line curve 11 anddotted line curve 0 show the relation between the amount of aluminumsalts added and interlamination resistances of insulating coatingsformed on the surface of electrical steel sheets. The interlaminationresistances of the insulating coatings each having a thickness of 0.8 to1.2 1. were plotted in function of the amount of aluminum in thealuminum salts. In order to obtain the insulating coatings on thesurface of an electrical steel sheet containing 1.2% by weight ofsilicon and having a thickness of 0.5 mm., this steel sheet is uniformlycoated with an aqueous solution containing 100 cc. of water, g. of Mg(HPO and 3 g. of CrO and added with various amounts of aluminum salts. Theinsulating coatings thus obtained are baked in a furnace at atemperature of 450 C. for one minute.

As seen from FIG. 1, more than 0.4 part by weight based on aluminum ofaluminum compound added is remarkably effective, but more than 1.5 partsby Weight is substantially non-effective.

In FIG. 1, the curves a and b show changes of the interlaminationresistances in dependence with additions of aluminum hydroxide A1(OH)and aluminum nitrate nonahydrate Al(NO -9H O, respectively, the more theamount of aluminum is present, the higher the interlamination resistancebecomes. In FIG. 1, the curve 0 shows the change of the interlaminationresistance of a grain oriented silicon steel sheet covered with a heatresistance glass film on which is formed an insulating coating with theaid of a reagent containing aluminum nitrate nonahydrate Al(NO -9-H O.As seen from the curve 0, in this case the interlamination resistance becomes considerably increased. But, the correlation between theinterlamination resistance and the amount of aluminum salt added issubstantially the same as that shown by the curves a and b where thereagent is directly coated on the abovementioned steel sheet.

The solubility of the amorphous aluminum hydroxide in an aqueoussolution of the monobasic magnesium phosphate is comparatively small.For example, more than 6 g. of amorphous aluminum hydroxide cannot bedissolved into 100 cc. of aqueous solution of the monobasic magnesiumphosphate containing 12 g. of chromic anhydride. It is not preferable toadd the amorphous aluminum hydroxide in an amount not soluble in theaqueous solution of the monobasic magnesium phosphate since suchaddition results in formation of a rough insulating coating withoutluster.

Moreover, addition of more than 30 parts by weight of aluminum nitrateis not preferable, because such addition causes the surface of the steelsheet and the treating liquid coated thereon to react each other toproduce spot patterns which are easily rustable, thereby degrading theappearance and the value of the steel sheets available in market.

Under the above reasons, the amount of baked on aluminum of aluminumcompounds added in accordance with the invention is limited to 0.4 to2.1 parts by weight.

FIG. 2 shows the relation between the amount of aluminum compounds addedand adhesive properties of the insulating coating. FIG. 2 was plottedfrom the re sults obtained by measuring the adhesive properties of theinsulating coatings formed by reagents comprising cc. of water, 20 g. ofMg(H PO.;) 0 to 6 g. of CrO and O to 8 g. of aluminum nitrate.

As seen from FIG. 2, the addition of Al(NO -9H O causes an improvementof the adhesive property of the insulating coating and the addition ofCrO is effective to further improve the adhesive property of theinsulating coating.

Relationships substantially the same as those shown in FIG. 2 wereobtained by using Al(OH) or a mixture of and instead Of Sing A].(NO3)3.

The addition of C1-C serves not only to improve the adhesive property ofthe insulating coating, but also to improve the corrosion resistance ofthe insulating coating. For the latter purpose, it is necessary to addmore than 5 parts by weight of Cr0 with respect to 100 parts by WeightOf Mg(H PO Moreover, CrO is effective to make the insulating coatingsmooth and vitreous. When using Al(OH) it is possible to make itssolubility higher by decreasing the pH of the reagent, while when usingAl(NO CrO is capable of prohibiting the above mentioned reaction betweenthe steel sheet surface and the reagent coated thereon and hencepreventing occurrence of the spot patterns.

The addition of more than 30 parts by weight of CrO makes the reductionof CrO by means of the baking step difficult, and hence soluble Cr+remains in the insulating coating to make it hygroscopic in air, thusliquefying and falling off the insulating coating from the steel sheet.

In the above mentioned composition ratio of the reagent, it is mostpreferable to use a range of 0.7 to 2.1 parts by weight based onaluminum of aluminum salts and 10 to 20 parts by weight of CrO The useof 6 to 25% concentration of the solution based on Mg(H PO permits ofobtaining good workability and forming an excellent insulating coatingin an industrially easy manner.

If necessary, proper amount of pulverized heat resistant substance suchas colloidal silica, mica or surface active agent may be added to thereagent according to the invention.

The insulating coating formed by the reagent according to the inventionand having a thickness of 0.3 to 2.5 may have excellent properties suchas adhesive property, interlamination resistance etc. The insulatingcoating having a thickness less than 0.3 is low in interlaminationresistance and inferior in corrosion resistance, while the insulatingcoating having a thickness more than 3.0;/. becomes opaque viridescentin color and flaky in shape, so that the insulating coating is liable tobe flaked away from the surface of electrical steel sheets. But, inaccordance with the invention it is possible to apply a reagentaccording to the invention again to the insulating coating which hasbeen formed on the steel sheets and baked to obtain an insulatingcoating having a thickness which could not be obtained by one treatment,thus providing an insulating coating having an extremely highinterlamination resistance and adhesive property. The composition,concentration and baking temperature of the reagent reapplied to theinsulating coating may be the same as those of the reagent applied atthe first time.

One of the advantages of the invention is that the reagent according tothe invention after having been coated may be baked at a comparativelylow temperature for a short time. If the baking temperature is lowerthan 300 C., the insulating coating thus baked becomes hygroscopic. Ithas been found out that if the insulating coating is baked in a furnaceat a temperature of 300 to 600 C. for 30 to 120 seconds the insulatingcoating thus baked becomes non-hygroscopic and is particularly adaptedfor use in various electrical fields. FIG. 3 shows relationships betweenbaking times at 450 C., for example, and amounts of aluminum compoundsadded. FIG. 3 was plotted by observing the surface condition of a steelsheet having a thickness of 0.5 mm. and coated with a reagent comprising100 cc. of water, 20 g. of Mg(H PO 3 g. of CrO and various amounts ofaluminum compounds and baked in a furnace at a temperature of 450 C. Thesurface condition of the steel sheet was observed after hours by thesalt spray test. If the baking -step is not sufficient, the insulatingcoating becomes hygroscopic and flows off the steel sheet, while if thebaking step is excessive, the adhesive property and corrosion resistanceof the insulating coating become deteriorated.

In view of the above mentioned results, the use of Al(NO as the aluminumcompounds can decrease the baking time by the order of to seconds.

The grain oriented silicon steel sheet for use in a large typetransformer has recently been coated with an insulating coating adoptedby steel sheet makers without being subjected to the varnish bakingstep. Such silicon steel sheet is required not only to have a highinterlamination resistance, but also to have a high break down voltage.The insulating coating formed by the reagent according to the inventionhas a remarkably improved break down voltage as compared to otherphosphate coatings and hence can be applied to the grain orientedsilicon steel sheet for use in the large type transformers.

FIG. 4 shows a circuit arrangement for measuring the break down voltage.A grain oriented silicon steel sheet coated on the surface thereof withglass film is further subjected to be coated on the glass film with thereagent according to the invention and baked thus providing aninsulating coating having a thickness of 2a. The thus treated sheet isannealed at a temperature of 800 C. in an N; atmosphere. The break downvoltages of the insulating coating thus obtained and measured by thecircuit arrangement shown in FIG. 4 are shown in the following table.

Amount of aluminum compound Break down added (based on Al):

In FIG. 4, 1 designates a 220 v. alternating current supply source, 2 avoltage regulating transformer, 3 an ammeter, 4 a resistor, 5 a circuitbreaker, 6 a voltmeter, 7 an electrical steel sheet to be tested, 8 aninsulating coating formed by a reagent according to the invention, 9 ameasuring electrode having a diameter of 20 mm. and made of brass and 10a measuring drill rod.

As seen from the above table, the addition of the aluminum compoundserves to remarkably improve the break down voltage of the insulatingcoating.

The insulating coating formed on the surface of electrical steel sheetsby the reagent according to the invention has various properties otherthan those described above, which read as follows.

The insulating coating provides a colorless glass-like film which islustrous and beautiful in appearance and so hard that it is difiicultfor it to be scratched.

The insulating coating is thin and smooth so that the space factor ofthe steel sheets having the insulating coating formed thereon issubstantially the same as that of the steel sheets formed without theinsulating coating.

The insulating coating has an excellent corrosion resistance andprevents formation of rust and has an excellent antichemical propertyand is stable against Freon gas and not damaged by transformer oil andpunching oil.

The insulating coating is not flaked oil from the steel sheet whensubjected to the stress relief annealing step and thus maintains arelatively high interlamination resistance even after the annealingstep. The insulating coating does not contain any organic compound andhence has a good weldability and does not cause blow holes after beingsubjected to a TIG-arc welding.

An electrical steel sheet containing 1.2% by weight of silicon andhaving an insulating coating formed thereon by the reagent according tothe invention could be punched through by a punching die made of steeland having a clearance of 40,14 to obtain discs over one hundred andninety thousand punchings until burr formed along the periphery of thedisc obtained reaches to a height of 100 Such number of punching isabout two times larger than that'of the electrical steel sheet coatedwith conventional phosphate insulating coating and then baked.

The reason why the addition of aluminum nitrate and/ or amorphousaluminum hydroxide to an aqueous solution consisting of monobasicmagnesium phosphate and chromic anhydride is capable of considerablyimproving the film properties such as interlamination resistance,adhesive property, etc., of the insulating coating is not clear atpresent. But, the presence of N0 radical is no longer detected in aninsulating coating formed by a reagent containing aluminum nitrate, andaluminum nitrate and amorphous aluminum hydroxide are decomposed at 250C. and 300 C., respectively, and hence it is believed that aluminumnitrate and amorphous aluminum hydroxide would be converted into a sortof oxides or hydroxy oxide. 4

But, the addition of alumina (A1 0 instead of these aluminum compoundsresults in a rough coating whose interlamination resistance and adhesiveproperty etc., are not improved and whose punchability becomesconsiderably deteriorated.

If aluminum sulfate is used as a soluble aluminum compound, aluminumsulfate is not decomposed at a temperature less than 1,000" C. so that80.; radical remains in the coating thus deteriorating its corrosionresistance and not improving its insulating property and adhesiveproperty.

AlCl is also soluble in the magnesium phosphate, but the reagentcontaining AlCl when in contact with the steel sheets, immediatelybegins to react with them and spot-like patterns of tribasic phosphateare produced throughout the surface of the coating. Thus, after thesteel sheets have been subjected to the baking step the coating is badin appearance, corrosion resistance and adhesive property and cannot beused as an insulating coating.

-If the monobasic magnesium phosphate contains a free phosphoric acid, aportion of added amorphous aluminum hydroxide reacts with the freephosphoric acid to produce aluminum phosphate with the result that thedesired film properties cannot be obtained. Moreover, if the monobasicmagnesium phosphate contains free phosphoric acid, the above mentionedspot-like patterns of tribasic phosphate are liable to be produced onthe coating so that it is preferable to make the amount of the freephosphoric acid in the monobasic magnesium phosphate as small aspossible.

The invention will now be described with reference to examples.

EXAMPLE 1 To 100 l. of phosphoric acid was added 200 l. of water andthen added gradually 24.2 kg. of magnesium oxide to produce about 30%monobasic magnesium phosphate solution. The amount of free phosphoricacid in the solution was measured. If the free phosphoric acid contentwas high, a small amount of magnesium carbonate was added to saidsolution so as to adjust the free phosphoric acid content.

6 kg. of chromic anhydride 7.5 kg. of aluminum nitrate (Al(NO -9H O) in100 1. of water were added to 100 l. of said solution and dissolvedtherein. The reagent thus obtained was uniformly coated on the surf-aceof an electrical steel sheet containing 1.2% by weight of silicon withthe aid of grooved rubber rolls. The steel sheet thus coated with thereagent was placed in a furnace and baked at a temperature of 450 C. forone minute.

Various properties of the coating thus obtained were measured, whoseresults are as follows.

INTERLAMINATION RESISTANCE [ASTM A344, 35 kgJcmfl] Test. After annealing(N 1, Thickpiece Before annealing 750 0., 3 hours) ness No. 1.-- 56.7trawl/lamination". 19.4 Q-cm. /laminatin: 1. 0;; N o. 2. 31.7Mini/lamination." 7.9 flcmflllamination 0. 6

CORROSION RESISTANCE salt spray test at 35 C.

No. 1-No rust gathers during 20 hours. No. 2--No rust gathers during 12hours.

Weld ability Electrical steel sheets are laminated one upon the otherand the side end surface of the laminated sheets is coated with apunching oil available in market as a trade name of Nihon Kosakuyu No.6300. Then, the laminated sheets were subjected to TIG-arc welding usinga mug sten electrode having a diameter of 1.6 mm. under Ar 5 l./min., acurrent of 120 a. and travel speed of the electrode of 500 mm./min. Thebeads obtained of the test pieces Nos. 1 and 2 were beautiful oneswithout blow holes and serpentine tumings.

Resistance against Freon Electrical steel sheets were immersed intoFreon 22 enclosed in a hermetically sealed vessel made of stainlesssteel at a temperature of 40 C. for 1700 hours. Thus, the resistanceagainst Freon of the steel sheets was measured. The appearance of thesteel sheets was not changed owing to occurrence of rust etc., and theproperties of the steel sheets were not also changed.

EXAMPLE 2 INTERLAMINATION RESISTANCE [ASTM A344, 35 kg.lcm.

Test. After annealing (N Thickpiece Before annealing 750 0., 3 hours)11653 N o. 3..." 42.6 wand/lamination. 9.2 SZ-cm.-/lamination 0. 8s

CORROSION RESISTANCE 5% salt spray test at 35 C.

No. 3No rust occurs during 25 hours.

Weldability and anti-Freon resistance were the same as those obtained inthe Example 1.

EXAMPLE 3 IN TERLAMINATION RESISTANCE [ASTM A344, 35 kgJcmfl] Test Afterannealing piece Before annealing (N2, 800 0., 5 hours) BREAK DOWNVOLTAGE After annealing163 v.

The description and examples given above are intended to illustrate thebest mode of performing the invention. It is apparent that manymodification thereof may occur to those skilled in the art, which willfall within the scope of the following claims.

What is claimed is:

1. A reagent for forming an insulating coating on the surface ofelectrical steel sheets comprising an aqueous solution consisting ofparts by weight of monobasic magnesium phosphate and 5 to '30 parts byweight of chromic anhydride, and 0.4 to 2.1 parts by weight based onaluminum of aluminum nitrate and/or amorphous aluminum hydroxide.

2. A reagent as claimed in claim 1, wherein said aqueous solutionconsists of 100 parts by weight of monobasic magnesium phosphate havinga concentration of 6 to 25% by weight and 10 to 20 parts by weight ofCrO and added with 0.7 to 2.0 parts by weight based on aluminum ofaluminum nitrate and/0r amorphous aluminum hydroxide.

3. A reagent as claimed in claim 1 and further comprising a pulverizedheat resistance substance such as colloidal silica, mica or surfaceactive agent.

4. A method of forming an insulating coating on the surface ofelectrical steel sheets comprising the steps of coating the surface ofan electrical steel sheet with an aqueous solution consisting of 100parts by weight of monobasic magnesium phosphate and 5 to 30 parts byweight of chromic anhydride, and 0.4 to 2.1 parts by weight based onaluminum of aluminum nitrate and/or amorphous aluminum hydroxide to forman insulating coating on the surface of the electrical steel sheets, andbaking the insulating coating thus obtained in a furnace at atemperature of 300 to 600 C. for 30 to seconds.

5. A method as claimed in claim 4, wherein said aqueous solution isapplied to the said insulating coating formed on the steel sheets andthen baked.

6. A method as claimed in claim 4, wherein said insulating coating isformed on a heat resistant glass film covered on the surface of grainoriented silicon steel sheets.

7. An insulating coating formed on the surface of electrical steelsheets by the reagent as claimed in claim 1 and having a thickness of0.3; to 2.5g.

8. An insulating coating formed by the method as claimed in claim 5 onthe insulating coating which has been formed on the surface ofelectrical steel sheets.

9 10 9. An insulating coating formed by the method as 3,151,997 10/1964Harvey 148113 X claimed in claim 6 on the heat resistant glass filmcovered 3,207,636 9/1965 Wada et a1. 148-616 on the surface of grainoriented silicon steel sheets. 3,395,027 7/ 1968 Klotz 1486.16 X

References Cited 5 RALPH S. KENDALL, Primary Examiner UNITED STATESPATENTS C. WESTON, Assistant Examiner 2,753,282 7/1956 Perry 1486.16 US.Cl. X.R.

3,151,000 9/1964 Schmidt et a1 148113 X 1486.2, 113

